A virus infecting the haptophyte Phaeocystis pouchetii (Hariot) Lagerheim was isolated from Norwegian coastal waters in May 1995 at the end of a bloom of this phytoplankter. The virus was specific for P. pouchetii because it did not lyse 10 strains of P. globosa Scherffel, Phaeocystis sp., and P. antarctica Karsten. It was a double‐stranded DNA virus, and the viral particle was a polyhedron with a diameter of 130–160 nm. The virus had a main polypeptide of about 59 kDa and at least five minor polypeptides between 30 and 50 kDa. The latent period of the virus when propagated in cultures of P. pouchetii was 12–18 h, and the time required for complete lysis of the cultures was about 48 h. The burst size was estimated to be 350–600 viral particles per lysed cell.

In this experimental study we investigated how growth conditions and physiological status of the marine haptophyte Phaeocystjs pouchetii affect its interaction with the lytic virus PpVO1. A simple mathemat~cal model describing the infection, the lysis of algae and the production of new viral particles was developed as an aid for analyzing and understanding the interaction between host and virus. P. pouchetij was susceptible to virus infection in all stages of growth. Nutrient or light limitation of algal growth did not inhibit viral reproduction and cell lysis. Neither the infectivity of the progeny viruses produced nor the length of the lytic cycle appeared to be affected by the host cells' growth condltion. However, possible effects may have been obscured by low accuracy of infective virus counts or by low sampling frequency. The host cells' growth conditions did have a significant impact on burst size. A maximum of 510 viruses produced per infected host cell was found in exponentially growing cultures, while low burst size (minimum 15) was found in stationary phase cultures, in nutrient depleted cultures and in light limited cultures.

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In this experimental study we investigated the effect of viral infection on primary production and carbon flow in a phytoplankton-DOC-bacteria food chain during viral lysis of the phytoplankton population. The phytoplankter host-virus system used was Phaeocystis pouchetii (Pryrnnesiophyceae) and the virus PpVO1. Viral infection allowed primary production in the cells to continue throughout most of the lytic cycle. In non-infected algal cultures, net production of DOC and bacterial biomass was low and at the end of the experiment the DOC concentration was 10 to 2096, and the bactenal biomass 0.5 to 4 % of the algal carbon biomass. The amount of DOC released during viral lysis of the algal cells implies that the enhre algal biomass was converted to DOC. Growth of bacteria succeeding cell lysis and release of DOC in virus infected cultures demonstrated that the net effect of the virus infection was an efficient conversion of algal biomass into bacterial biomass.

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